Method and apparatus for analogue processing of the signal emitted by a particle detector

ABSTRACT

Devices and methods for processing a signal emitted by a particle detector in which the methods include detecting in the signal the portions where the signal is greater than a predetermined value V 1,  measuring the maximum value Vmax reached by the signal in each of the portions, and associating with each of the portions an analog quantity Q which, at least in a predetermined range of values DELTA V 1  of the maximum value Vmax, is an increasing function of (Vmax−V 1 ).

PRIORITY CLAIM

This application claims priority to French Patent Application No.0017220, filed Dec. 28, 2000.

TECHNICAL FIELD

The present invention relates to the analysis of a flux of particlesreceived by a particle detector during a given interval, for examplewith the aim of counting these particles or measuring their energy. Itrelates more particularly to method of analysis using analogueprocessing of the signal emitted by the said detector, as well as thedevices and apparatus intended for implementing these methods.

BACKGROUND

The detectors considered in the present invention are known detectors,whether of the point or matrix type, and regardless of the materials,semiconducting or otherwise, of which they are composed. The signalsemitted by these detectors can either be electric currents, or they canbe of a physical nature and can be converted to electric current in aknown manner. It will simply be assumed that reception of a particle bythe detector triggers an output signal having the form of a pulse of acertain width and the maximum amplitude of which is representative ofthe energy of this particle.

In the rest of the description, for the purpose of clarity, referencewill be made to the detection of “photons” (more particularlymeasurement of the characteristics of electromagnetic radiation), but itis to be borne in mind that the invention is completely independent ofthe nature of the particles detected.

An important factor limiting the quality of signal processing is thebackground noise which is always present in the current emitted by thedetector. This background noise includes at least two components. Thefirst component is the “dark current”, more particularly, thefluctuating current, of thermal origin, emitted by the detector evenwhen it is not receiving any photons. The second component is the“transient decay current”, more particularly the fluctuating currentthat is manifested for a certain time after reception of a photon by thedetector; in detectors using semiconducting materials, this transientdecay current is due in particular to crystal imperfections in thesematerials.

Let us examine the effects of this background noise on the accuracy ofmeasurements carried out by the known methods.

If for example a measurement system based on integration is used, bymeans of which the total energy of the radiation received by thedetector during a predetermined time is measured, the current from thedetector is integrated over this time. A faithful representation of theenergy of the photons received then requires taking into account all ofthe current produced, including the low values: use of a threshold ofdetection of this current would therefore be detrimental, in that itwould cause a loss of information. However, the measured currentincludes, as explained above, a component due to the background noise,to which an exact value cannot be ascribed because of the fluctuationsdue to thermal drift and to the transient decay current, and because ofthe random noise associated with this component. In the known systems,the current due to the background noise is integrated in the measuredvalue, and then a quantity that is only an estimated average value ofthe effect of the background noise is subtracted, in order to obtain therepresentative value of the radiation energy.

As another example, when a measurement system based on counting is used,by means of which the number of photons of energy E above a threshold E₂received by the detector during a predetermined time is measured, asuitable device (for example a bistable circuit) is triggered when thesignal exceeds a certain threshold value corresponding to E₂, and thesaid device is reset when the signal falls below this threshold value.Admittedly, there is then nothing to prevent the placement, at thedetector output, of a system for filtering the continuous component fromthe background noise. However, the problem is that it is not possible todistinguish an increase in current due to the arrival of a photon fromthe increase in current caused by a fluctuation of the background noise,unless the said threshold value is set high enough so that thefluctuations can practically never exceed it. In addition, in theseconventional measurement systems, the bistable circuits or similardevices produce parasitic coupling. In practice, this threshold cannottherefore be set very low.

SUMMARY

The invention applies to any field where the analysis of a flux ofparticles, and in particular their counting or measurement of theirenergy, can be useful, for example in the case when these particles arephotons, in radiology, in fluoroscopy or in video imaging. It isparticularly suited to fields requiring a method of signal processingwhich, though of high quality (in the sense that the said method permitsvery accurate measurements of the flux), does this using a device ofsmall size; this is in particular the case when this device is composednot of a single detector (pixel), but of a matrix of pixels, because thesize of the electronic apparatus used is then limited by the spacing ofthe pixels.

A subject of the invention is therefore methods, and relatively compactdevices implementing these methods, which are intended to reduce thesensitivity of measurements of particle flux, on the one hand tofluctuations of the background noise present in the signals emitted bythe detectors, and on the other hand to disturbances caused by themeasurement electronics.

With this aim, the invention includes a method of processing the signalproduced by a particle detector, the said method being characterized inthat the portions where the signal is above a predetermined value V₁ aredetected in the the signal, the maximum value V_(max) reached by thesignal in each of the said portions is measured, and an analoguequantity Q which, at least over a predetermined range of values ΔV₁ ofthe said maximum value V_(max), is a, for example linearly, increasingfunction of (V_(max)−V₁), is assigned to each of the said portions.

In fact, the invention exploits the fact that, in conventionaldetectors, the “peak” of each current pulse caused by an incidentparticle is proportional to the energy of that particle, or is at leastrepresentative of this energy (assuming for the purpose of clarity thatthe the pulse has positive values: the reader will easily transfer thecharacteristics of the invention to the case when negative values aremeasured). The method according to the invention thus only takes intoconsideration this peak (in the form of V_(max)), without taking intoaccount the rising part and the falling part of each pulse, and to aneven lesser extent the value of the signal between the pulses, so thatthe effect of fluctuations of the background noise is felt only for thebrief duration of these pulses, in the course of which the measurementsare carried out. This leads to an appreciable improvement of the qualityof the measurements relative to the conventional methods.

With regard to the practical choice of the detection threshold V₁, it isclear that for this thresholding to be effective, generally a value ofV₁ must be chosen that is above the average level of the backgroundnoise (or must be chosen as positive if, at the detector output, thecontinuous component is filtered from the background noise). This beingso, the higher the value of V₁, the more the fluctuations of thebackground noise are avoided. However, the presence of this threshold V₁prevents the detection of photons, the energy of which (if applicable)is lower than the energy E₁ associated with a voltage pulse peaking atV₁; consequently, the higher the value of V₁, the larger will be theenergy band for which photons of energy belonging to this band will notbe able to be detected. Therefore the value of V₁ must be chosencarefully, using these principles, as a function of the particularapplication.

The variation, according to the invention, of the analogue quantity Qover the range of values ΔV₁ naturally makes it possible to take accountof the energy variations of the photons received, and in a way that candiffer from one application of the invention to another. In photometry,for example, it will be possible to prefer a linear behaviour of theanalogue quantity Q as a function of the energy E of the photon. Forphoton counting, the increasing (but not necessarily linear) functionQ(E) means that it is possible to implement “progressive thresholding”about a predetermined energy, as explained below.

According to particular characteristics of the invention, there isassigned to each of the said portions an analogue quantity Q which is anincreasing function, for example linear, of (V_(max)−V₁) if the maximumvalue V_(max) is below a second predetermined value V₂, and remainsconstant at its value for V_(max) equal to V₂ if the maximum valueV_(max) is above this second value V₂, at least over a predeterminedrange of values ΔV₂ of the said maximum value V_(max).

These particular characteristics are very advantageous when theinvention is applied to photon counting, which consists, it will berecalled, of measuring the number of photons with energy E above athreshold E₂ received by the detector during a predetermined time. Infact, an analogue quantity Q proportional to (E−E₁) will then beassigned to every photon of energy E between E₁ and E₂, and an analoguequantity Q₂ proportional to (E₂−E₁) will be assigned to every photon ofenergy E greater than E₂. In this way, progressive thresholding aboutE=E₂ is implemented.

It will be noted that this progressive thresholding has the advantage,in comparison with very abrupt thresholding, of making it possible totake account of peaks, the real energy of which is minimized by aparticularly low instantaneous level of background noise: in fact, theprobability that a particle with apparent energy (as detected) below thecounting threshold that has been adopted still has a real energy abovethis threshold increases as the difference between this apparent energyand the counting threshold decreases. This progressive thresholding cantherefore be understood as the application, to the apparent energy ofthe observed peaks of a likelihood coefficient that approaches 1 as thisapparent energy approaches E₂. In reality, the counting threshold (abovewhich it is desired to characterize the particles) is below E₂ (but itis not necessary to know it) to the extent that there may also be peakswith apparent energy greater than their real energy because of aparticularly high instantaneous level of the background noise: accordingto this approach, E₂ is the energy level for which it is considered thatit is certain, regardless of the instantaneous value of the backgroundnoise, that it is indeed a particle that has a real energy at leastequal to the above-mentioned counting threshold.

Furthermore, this progressive thresholding makes it possible to takeaccount of the fact that, in the analysis of a physical phenomenon, theenergy transition between the significant particles and those that arenot, is not necessarily abrupt, and the particles with energy close tothe threshold can contribute to the phenomenon which it is beingattempted to characterize; progressive thresholding can then be analysedas assignment of a coefficient of efficiency of the peaks that movescloser to 1 as the energy level moves closer to E₂.

These two approaches can of course be combined, and the choice of thethresholds and of the slope of the rising portion makes it possible totake best account of what it is desired to characterize; this choice canbe made, for example, following tests conducted in accurately knownconditions. It must be understood here that the linear form of such arising curve is particularly practical, especially because of the smallnumber of coefficients to be chosen, but that other forms are possible,in order to take best account of the results that are expected fromprogressive thresholding.

This progressive thresholding makes it possible to position the levelabove which the signal is measured, very close to the level of thebackground noise: in fact, a very low, but non-zero, likelihoodcoefficient is applied to the low peaks, taking account of theprobability that this low peak is representative not of a fluctuation ofthe background noise but of a particle that should be accounted for.

It can therefore be seen that, relative to the conventional methods, theinvention offers the advantage of counting photons very accurately evenwhen the energy E₂ is very low. An additional advantage is the absenceof parasitic signals such as those produced, in conventionalbistable-circuit counters, by the switchings of this bistable circuitassociated with incrementing of the counter.

According to even more particular characteristics of the invention,there is assigned to each of the said portions an analogue quantity Qwhich is an increasing function of (V_(max)−V₁) if the maximum valueV_(max) is below a second predetermined value V₂, remains constant atits value for V_(max) equal to V₂ if the maximum value V_(max) isbetween this second value V₂ and a third predetermined value V₃, and isa decreasing function of V_(max) if the maximum value V_(max) is abovethis third value V₃, at least over a predetermined range of values ΔV₃of the maximum value V_(max).

Owing to these arrangements, it will be possible in particular to obtaina function Q(E) that is tooth-shaped, or approaches a Gaussian curve. Itwill thus be possible to favour, in the radiation received, the photonsbelonging to a relatively narrow energy band, these photons beingparticularly significant in the application envisaged.

According to another aspect, the invention relates to various devices.

Firstly, it thus relates to a device for processing the signal producedby a particle detector, the device comprising a conversion unit that isable to convert any pulse of current emitted from the said detector intoa voltage pulse V, an analogue circuit including

-   -   an electric charge storage device D₃, a first electric charge        receiver D₁ that can be fed by the charge storage device D₃ in a        manner controllable by means of the voltage V, and a second        electric charge receiver D₂ that can also be fed by the charge        storage device D₃ in a manner controllable by means of the        voltage V, and an apparatus for measuring the electric charge Q        contained in the second charge receiver D₂,    -   the analogue circuit being designed in such a manner that each        voltage pulse V produces the following effects successively        within the device: the charge storage device D₃ is isolated from        the first charge receiver D₁, the charge storage device D₃ is        connected to the second charge receiver D₂ when the voltage V        exceeds a predetermined value V₁, an electric charge Q that is        an increasing function of (V−V₁) passes from D₃ to D₂, the        connection between D₃ and D₂ is cut when the voltage V begins to        decrease after reaching a maximum value V_(max), and D₃ is        reconnected to D₁ which restores the lost charge Q in D₃.

Secondly, the invention also relates to a device for processing thesignal produced by a particle detector, the device comprising aconversion unit that is able to convert any pulse of current emittedfrom the detector into a voltage pulse V, an analogue circuit includinga charge storage device M₂, a first electric charge receiver D₁ that canbe fed by the charge storage device M₂ in a manner controllable by meansof the voltage V, and a second electric charge receiver D₂ that can alsobe fed by the charge storage device M₂ in a manner controllable by meansof the voltage V, and an apparatus for measuring the electric charge Qcontained in the second charge receiver D₂, the analogue circuit beingdesigned in such a manner that each voltage pulse V produces thefollowing effects successively within the device:

-   -   the charge storage device M₂ is isolated from the first charge        receiver D₁,    -   the charge storage device M₂ is connected to the second charge        receiver D₂ when the voltage V exceeds a first predetermined        value V₁,    -   an electric charge Q proportional to (V−V₁) passes from M₂ to D₂        if the voltage V does not exceed a second predetermined value        V₂, or proportional to (V₂−V₁) if the voltage V exceeds the said        second value V₂, the connection between M₂ and D₂ is cut when        the voltage V begins to decrease after reaching a maximum value        V_(max), and M₂ is reconnected to D₁ which restores the lost        charge Q in M₂.

Thirdly, the invention also relates to a device for processing thesignal produced by a particle detector, the device being characterizedin that it comprises two circuits similar to that briefly described forthe second device, and both receiving the voltage pulse V emitted from aconversion unit, the parameters of these two circuits being controlledindependently of one another, and an analogue subtractor that is able toproduce an output signal equivalent to the difference Q between therespective analogue charges Q′ and Q″ transferred to the respectivesecond charge receivers D′₂ and D″₂ contained in the circuits.

Finally, the invention fourthly relates to a device for processing thesignals produced by a set of particle detectors, the device beingcharacterized in that at least one of these signals is processed bymeans of a device such as those briefly described above.

For each of these devices, the measurements are carried out by samplingand reading this analogue quantity Q at predetermined points of time.Since this quantity depends on V_(max), it is properly representative ofthe energy of the photon giving rise to the pulse. In particular, inorder to count the photons received since the preceding measurement, itis sufficient to divide Q by Q₂.

The advantages offered by these devices are therefore essentially thesame as those offered by the methods according to the invention, but itwill be noted in addition that they can easily be constructed usingconventional semiconductor components, as will be shown in the detaileddescription given below, hence the small overall size of these devices,as well as a low cost of manufacture. These properties arise inparticular from the fact that, according to the invention, processing ofthe signal from the detector is purely analogue. Naturally, in certainapplications it will prove useful to connect an analogue-digitalconverter to a device according to the invention, in order to permitdigital processing of the information obtained, especially ifconstraints of cost and overall size are secondary in the applicationenvisaged.

Finally, the invention relates to various apparatus for analysing a fluxof particles incorporating at least one device such as those describedbriefly above.

BRIEF DESCRIPTION OF THE DRAWING

Other aspects and advantages of the invention will become evident onreading the detailed description, given below, of particular embodimentsgiven as non-limitative examples. This description refers to theappended drawings, in which:

FIG. 1 schematically represents a device according to a first embodimentof the invention,

FIGS. 2 a to 2 d represent the main stages in the operation of thedevice shown in FIG. 1,

FIG. 3 schematically represents the relationship between the energy E ofan incident photon and the analogue quantity Q that is associated withit, when using the device shown in FIG. 1,

FIG. 4 schematically represents a device according to a secondembodiment of the invention,

FIGS. 5 and 6 represent the main stages in the operation of the deviceshown in FIG. 4,

FIG. 7 schematically represents the relationship between the energy E ofan incident photon and the analogue quantity Q that is associated withit, when using the device shown in FIG. 4,

FIG. 8 schematically represents a device according to a third embodimentof the invention, and

FIGS. 9 a to 9 d schematically represent the relationship between theenergy E of an incident photon and the analogue quantity Q that isassociated with it, when using the device shown in FIG. 8 according tofour different settings, presented here as examples.

DETAILED DESCRIPTION

FIG. 1 represents, according to a first embodiment of the invention, adevice 100 that is intended for processing the signals emitted by aphoton detector 2.

This detector 2 emits, in response to the arrival of a photon on itsreceiving surface, a pulse of current I. According to the invention,firstly this pulse of current I is converted to a voltage pulse V withthe help of a suitable conventional unit 1.

The continuous component is then, optionally, removed from the resultingsignal by means of a conventional filtering unit 5. It will be recalledthat this continuous component corresponds to the average value of thedark current and of the transient decay current leaving the detector 2,whatever this detector.

The signal is then processed by the analogue circuit 3. The voltagepulse V is applied directly to a diffusion zone D₁ which performs therole here of an electric charge receiver, and to the gate of a MOS(metal oxide semiconductor) transistor M₃. More precisely, in theembodiment represented, a transistor of the NMOS type was chosen for M₃,i.e. with conduction by electrons; the surface channel potential V* ofM₃ is therefore lower here than V by a certain amount ε.

Between M₃ and D₁, there is another diffusion zone D₃, which performsthe role of charge storage device, and another NMOS transistor M₁, thegate of which is maintained at a fixed potential V₁; the channelpotential V₁* of M₁ is less than V₁ by an amount close to ε.

Finally, after the transistor M₃, there is a final diffusion zone D₂which is intended to receive the analogue charge Q according to theinvention.

In order to carry out a measurement, this diffusion zone D₂ is brieflybrought to a predetermined fixed potential V_(R) (by closing and thenopening the switch S). The charge Q accumulated in D₂ then produces avoltage change that is read by a measuring apparatus 6 (for example acapacitor with voltmeter, or a ballistic galvanometer) supplying theoutput signal from the device V_(out).

FIG. 2 represents the main stages in the operation of the device shownin FIG. 1, showing schematically, for each stage, the relationshipbetween the potentials of D₁, D₂, D₃, and of the channels of M₁ and M₃,in the case when the value of V₁ is chosen in such a manner that thevalue of V₁*, equal to V₁−ε, is greater than the average value of thebackground noise, in the device 100 without the filtering unit 5 (or ispositive, if such a unit 5 is incorporated).

FIG. 2 a shows the values of these potentials in the absence of a pulsefrom detector 2. It can be seen in particular that the charges locatedin D₃ can flow into D₁, but not into D₂, because of the potentialbarrier presented by the channel of M₃.

Following the reception of a photon by the detector (or because of afluctuation of the background noise), the potentials of D₁ (moreparticularly V) and of M₃ (more particularly V* equal to V−ε) increasein concert. If the pulse is strong enough, the stage shown in FIG. 2 bis reached, where communication between D₃ and D₁ is cut.

If the pulse is strong enough, we then reach the stage shown in FIG. 2c, where the charges contained in D₃ can begin to flow into D₂. Thequantity of charges thus moved for a given voltage V depends on theparasitic capacity of D₃.

When the pulse V reaches the maximum V_(max) (FIG. 2 d), the chargemoved to D₂ has reached a certain value Q.

The voltage V then decreases, and M₃ immediately forms a potentialbarrier between D₃ and D₂, so that no additional charge flows to D₂.Therefore the charge Q preserves the value which it acquired at the peakof the pulse.

Finally, one returns to the situation in FIG. 2 a until a new pulsearrives. It is necessary to ensure, taking into account the practicalfrequency of arrival of the photons, that recharging of D₃ from D₁ isfast enough for the device to be ready for this new pulse.

FIG. 3 shows the shape of the function Q(E) (where E is the energy ofthe incident photon that gave rise to the voltage pulse V) associatedwith the device 100. This curve Q(E) is characterized by a detectionthreshold E₁ corresponding to a voltage pulse, the peak V_(max) of whichis equal to the voltage V₁. An increasing portion can then be observed,at least over an energy band of the photons ΔE₁ corresponding to a rangeof values ΔV₁ of V over which the circuit 3 behaves faithfully in themanner described above.

In the case when it is necessary, for the application envisaged, to havea linear increase, it will be possible for example to replace thediffusion zone D₃ with an NMOS transistor, the gate of which will bepolarized to a potential higher than the largest value expected forV_(max); or alternatively, it will be possible to connect the plate of acapacitor, the other plate of which is polarized to a fixed potential,to the diffusion zone D₃.

FIG. 4 represents, according to a second embodiment of the invention, adevice 200 that is intended for processing the signals emitted by aphoton detector 2.

This device 200 only differs from the device 100, and more precisely thecircuit 7 only differs from circuit 3, in the replacement of diffusionzone D₃ with an NMOS transistor M₂, the gate of which is brought to afixed potential V₂.

FIG. 5 represents the main stages in the operation of the device shownin FIG. 4 for a photon, the energy E of which is below E₂, where E₂corresponds to a voltage pulse, the peak V_(max) of which is equal tothe voltage V₂.

The operation of the device in this case is completely analogous to theoperation described with reference to FIG. 2. It is true that in thepresent device, when E is greater than E₁ (the value that corresponds toa voltage pulse, the peak V_(max) of which is equal to the voltage V₁),a stage is reached (starting from FIG. 5 b) where a certain charge Q₂ isisolated in the channel of M₂, which was not the case with the device inFIG. 1; but the value of this charge Q₂ has no effect on the functioningof the present device if E is lower than E₂.

Therefore let us examine, referring to FIG. 6, the main stages in theoperation of the device 200 for a photon, the energy E of which isgreater than the said value E₂.

The stages illustrated in FIGS. 6 a to 6 c are identical to therespective stages illustrated in FIGS. 5 a to 5 c. Then the charge Q₂flows as previously from M₂ to D₂, but, when V continues to increase, wereach a stage (V* is less than V₂*, where V₂* equal to V₂−ε, andtherefore V is greater than V₂) where this charge is exhausted.Therefore when voltage V reaches its maximum V_(max) (FIG. 6 d), thecharge deposited in D₂ is equal to Q₂ regardless of the value of thismaximum (assumed to be greater than V₂).

The return to the initial state (FIG. 6 a) is analogous to the return tothe initial state in the previous devices.

FIG. 7 shows the form of the function Q(E) associated with the device200.

This curve Q(E) is characterized by a detection threshold E₁, followedby a rising portion, the slope of which is determined by the capacity ofM₂. Then the function remains constant at a value Q₂ equal to Q(E₂), atleast over a photon energy band ΔE₂ corresponding to a range of valuesΔV₂ of V above V₂, over which the circuit 7 behaves faithfully in themanner described above.

FIG. 8 represents, according to a third embodiment of the invention, adevice 300 that is intended for processing the signals emitted by aphoton detector 2.

This device 300 comprises, in addition to a current-to-voltageconversion unit 1 and (optionally) a filtering unit 5, two circuits 7′and 7″ that are functionally similar to the circuit 7 of the device 200.The charges Q′ and Q″ accumulated respectively on D′₂ and D″₂ produce,after measurement in units 6′ and 6″, respective output signals V′_(out)and V″_(out) which are sent to an analogue subtractor 4, so that theoutput signal from device 300 is V_(out) equal to V′_(out)−V″_(out).

FIGS. 9 a to 9 d show the form of the function Q(E) (where Q is definedhere as being equal to (Q′−Q″)) associated with the device 300, forvarious values of V′₁, V′₂, Q′₂, V″₁, V″₂, and Q″₂.

In the case of FIG. 9 a, a common value (Q₀) is taken for Q′₂ and Q″₂,and equal capacities for M′₂ and M″₂ (so as to obtain equal slopes inthe rising part of the functions Q′(E) and Q″(E)), and in addition: V″₁equal to V′₂. We then obtain a triangular curve Q(E).

It may be desired to broaden the top of this curve, so that it becomesmore like a tooth, or a Gaussian curve. To do this (referring to FIG. 9b), it is sufficient to take V″₁ greater than V′₂.

By taking Q′₂ greater than Q″₂ (FIG. 9 c), a strobe pulse is obtainedwhich maintains a non-zero value of Q beyond E equal to E″₂.

Taking different capacities for M′₂ and M″₂ (FIG. 9 d), asymmetricslopes for the rising part and the falling part of Q(E) are obtained.

On the basis of these few examples, a person skilled in the art willeasily be able to choose from among the numerous possible settings ofparameters so as to obtain the required function Q(E) according to theapplication in question, among a large range of possible functionalforms.

Furthermore, it is self-evident that the devices shown in FIGS. 1, 4 and7 are deliberately simple examples of applications that are able tosupply the functions Q(E) shown in FIGS. 3, 7 and 9 respectively. Inpractice, a person skilled in the art will be able to modify them byknown techniques, so as to give them secondary advantages such asinsensitivity to parasitic noise, a rate of charge transfer through thedevice that is sufficiently fast, or stability of the current sources,amplifiers or transformers used.

Moreover, for the purpose of clarity, it was assumed in the abovedescription that the voltage pulse at the output of the current/voltageconverter is positive. In the case of negative pulses, a person skilledin the art will have no difficulty in adapting the devices described,for example by replacing the NMOS transistors with PMOS transistors(with hole conduction).

The invention was described above referring to the analogue charge Qaccumulated on a detector which can be either a single detector, or anindividual pixel within a multipixel detector, i.e. made up of a matrixor block of pixels.

In the case of a multipixel detector, there is certainly no reason why,if necessary, the analogue charges accumulated on several of thesepixels should not be summed. This summation offers for example aparticular advantage in the case of counting, if it is assumed that theenergy of the photons to be counted is, as is often the case, within arelatively narrow band positioned slightly above a counting thresholdE₂. The present invention then makes it possible to correct the countingerrors that might result from the fact that a certain photon arrivesbetween two pixels (which gives rise, at the output of each pixel, tosignals I₁ and I₂, the sum of which is equal to the signal I that wouldhave been produced if the said photon had arrived inside a singlepixel).

This is because, if a conventional device is used, neither of these twosignals I₁ and I₂ will be sufficient to trigger the counter associatedwith the respective pixel, so that the said photon will not be counted.Conversely, if a device according to the invention is used, an analoguequantity Q₁ proportional to I₁, and an analogue quantity Q₂ proportionalto I₂ will be recorded, so that the sum Q equal to Q₁+Q₂ will be roughlyequal to Q₂, and this photon will be counted correctly.

It will be noted in conclusion that the present invention can beconsidered overall from a different point of view from that presented inthe introduction. In fact, the many examples presented in detail aboveillustrate the fact that the signal processing according to theinvention leads to an analogue charge Q which represents, in apredetermined manner, the energy E of the incident photons. In otherwords, the function Q(E) performs the role of a “weighting function” bymeans of which we can ascribe, if necessary, a different “weight” toeach photon according to its energy. It has also been shown, in the caseof the weighting functions presented, how they can be obtainedconcretely by means of devices using conventional analogue electroniccomponents. Taking inspiration from these examples, a person skilled inthe art will be able to elaborate a suitable device for obtainingessentially any desired weighting function depending on the applicationenvisaged, or even a device offering possibilities for adjustmentallowing various forms of weighting curves to be obtained, suitable fora range of applications envisaged.

1. A method of processing a signal emitted by a particle detector, themethod comprising: detecting portions of the signal above apredetermined value V₁; measuring a maximum value V_(max) reached by thesignal in each of the portions; and assigning to each of the portions ananalogue quantity Q which, at least over a predetermined range of valuesΔV₁ of the maximum value V_(max), is an increasing function of(V_(max)−V₁); and assigning to each of the portions an analogue quantityQ which is an increasing function of (V_(max)−V₁) if the maximum valueV_(max) is below a second predetermined value V₂, and remains at aconstant value V_(max)=V₂ if the maximum value V_(max) is above thesecond predetermined value V₂, at least over a predetermined range ofvalues ΔV₂ of the maximum value V_(max).
 2. The method of signalprocessing according to claim 1, wherein the value of V₁ is at leastequal to the average level of background noise present in the signalemitted by the particle detector.
 3. The method of signal processingaccording to claim 1, wherein, over the range of values ΔV₁ of V_(max),the said analogue quantity Q is proportional to (V_(max)−V₁).
 4. Amethod of signal processing according to claim 3, further comprisingassigning to each of the portions an analogue quantity Q that isproportional to (V_(max)−V₁) if the maximum value V_(max) is below asecond predetermined value V₂, and that remains at a constant valueV_(max)=V₂ if the maximum value V_(max) is above the secondpredetermined value V₂, at least over a predetermined range of valuesΔV₂ of the maximum value V_(max).
 5. The method of signal processingaccording to claim 1 further comprising assigning to each of theportions an analogue quantity Q that is an increasing function of(V_(max)−V₁) if the maximum value V_(max) is below a secondpredetermined value V₂, and that remains at a constant value V_(max)=V₂if the maximum value V_(max) is between the second predetermined valueV₂ and a third predetermined value V₃, and that is a decreasing functionof V_(max) if the maximum value V_(max) is above the third predeterminedvalue V₃, at least over a predetermined range of values ΔV₃ of themaximum value V_(max).
 6. The method of signal processing according toclaim 1, wherein the particles detected by the particle detectorcomprise photons.
 7. A device for processing a signal produced by aparticle detector, the device comprising: a conversion unit configuredto convert any current pulse emitted from the detector into a voltagepulse V; an analogue circuit comprising an electric charge storagedevice, a first electric charge receiver which can be fed by theelectric charge storage device in a manner controllable by means of thevoltage pulse V, and a second electric charge receiver which can also befed by the charge storage device D₃ in a manner controllable by means ofthe voltage V; and an apparatus for measuring the electric charge Qcontained in the second charge receiver, wherein the analogue circuit isconfigured so that each voltage pulse V produces the following effectssuccessively within the said device: the charge storage device isisolated from the first charge receiver, the charge storage device isconnected to the second charge receiver when the voltage V exceeds apredetermined value V₁, an electric charge Q that is an increasingfunction of (V−V₁) passes from the charge storage device to the secondcharge receiver, the connection between the charge storage device andthe second charge receiver is cut when the voltage V begins to decreaseafter reaching a maximum value V_(max), and the charge storage device isreconnected to the first charge receiver which restores in the chargestorage device the charge Q that was lost.
 8. The device according toclaim 7, wherein the device is configured for processing the signalsproduced by a set of particle detectors, and wherein at least one of thesignals is processed by means of the device.
 9. The device according toclaim 7, and wherein the particles detected by the particle detectorcomprise photons.
 10. The device according to claim 7, wherein thedevice comprises a radiology apparatus.
 11. The device according toclaim 7, wherein the device comprises a video imaging apparatus.
 12. Thedevice according to claim 7, wherein the device comprises a fluoroscopyapparatus.
 13. A device for processing the signal produced by a particledetector, the device comprising: a conversion unit configured to convertany current pulse emitted from the particle detector into a voltagepulse V; an analogue circuit comprising a charge storage device, a firstelectric charge receiver configured to be fed by the charge storagedevice in a manner controllable by means of the voltage pulse V, and asecond electric charge receiver configured to be fed by the chargestorage device in a manner controllable by means of the voltage V; andan apparatus for measuring the electric charge Q contained in the secondcharge receiver, wherein the analogue circuit is configured so that eachvoltage pulse V produces the following effects successively within thesaid device: the charge storage device is isolated from the first chargereceiver, the charge storage device is connected to the second chargereceiver when the voltage V exceeds a first predetermined value V₁, anelectric charge Q proportional to (V−V₁) if the voltage V does notexceed a second predetermined value V₂, or proportional to (V₂−V₁) ifthe voltage V exceeds the said second value V₂, passes from the chargestorage device to the second charge receiver, the connection between thecharge storage device and the second charge receiver is cut when thevoltage V begins to decrease after reaching a maximum value V_(max), andthe charge storage device is reconnected to the first charge receiverwhich restores, in the charge storage device, the charge Q that waslost.
 14. A device for processing a signal produced by a particledetector, the device comprising: two analog circuits each configuredaccording to claim 13 and receiving a voltage pulse V emitted from aconversion unit, wherein parameters of the two analog circuits arecontrolled independently of one another; and an analogue subtractorconfigured to produce an output signal equivalent to the difference Qbetween respective first and second analogue charges transferred to tworespective second charge receivers contained in the two analog circuits.